1. Field of the Invention
The present invention relates to a magnetic field generating apparatus for a magnetic resonance imaging (MRI) system, in which permanent magnets and a steel element are arranged to produce a controlled magnetic field within a specified volume between magnetic pole surfaces.
2. Description of the Related Art
An important aspect in the medical field and applications of a magnetic resonance imaging (MRI) system is the uniformity of a magnetic field within the separation between two magnetic pole surfaces facing each other. As defined herein, the magnetic pole is a north or south pole of a magnet, such as an electromagnet, a permanent magnet or a superconductive magnet, or any magnet surface where the field flux lines are to be controlled. The magnetic field emanates from one magnetic pole surface and terminates at the other magnetic pole surface. Typically, the magnetic pole surfaces are flat in the central portion which is close to an object to be photographed, for example, the body of a patent.
The magnetic flux density is commonly labeled B and the magnetic field strength is conventionally labeled H (each being vector quantities having direction). The uniformity of the magnetic field depends in part upon the uniformity of the separation between the magnetic pole surfaces, the homogeneity and permeability of the pole material, and the pole correction (shim) method, i.e., adding or subtracting small rings or buttons of ions on the pole surface. For cylindrical or polygonal poles with flat surfaces the concentricity of the cylinders (or polygons), and the separation and parallelism of the pole surfaces, must be precisely controlled to produce a uniform field. Thus, the pole holding structure must be controlled to hold the two magnetic pole surfaces at a precise distance apart, at a precise concentricity, and at a precise angular orientation (usually parallel).
Also, such a pole holding structure must be constructed to accommodate other auxiliary magnets used to shim and confine the various magnetic fields. One such feature concerns the flux return path. There is a need for an efficient design of a structure which minimizes the volumes of permanent magnets and the material (such as steel) surrounding them while still providing a given uniform field. Another need is a structure that allows adjustment of the field strength of the individual poles, or "pole strength matching". Since the flux (B) lines have no end points--the lines form closed loops--use of materials, relative physical sizes and orientations of structures affecting any part of the flux lines must be controlled to provide a given field strength and homogeneity. The support and positioning structures are adjustable, allowing fine adjustment of the separation and parallelism of the pole surfaces. This is necessary, for example, to compensate for manufacturing tolerances.
Table 1 illustrates the comparison of three types of permanent magnetic circuits for MRI systems. In particular, characteristics of a tunnel-like magnetic system of the prior art, an ordinary magnetic system of the prior art and an embodiment of a magnetic system according to the present invention are tabulated in Table 1. From Table 1, it can be understood that the configuration becomes complicated to meet the requirements according to the development of a new technique.
TABLE 1 Type of magnet system Tunnel-like Ordinary magnet Novel magnet Characteristcs magnet system system system Magnetic circuit ##STR1## ##STR2## ##STR3## Size of mag- Small Medium Small netic circuit Tolerance for Small Large Medium magnetic properties Easiness for Difficult Easy Easy assembly Flux leakage Small Small Very small Temperature Large Medium Medium dependence Magnetic field No shielding Medium Large shielding Sensitivity for Large Medium Small steel Tuning of Impossible Easy Easy homogeneity at hospital
In magnetic systems where the field distribution is determined by the magnetic pieces alone, it is difficult to achieve accuracy with field errors in the order of 1/100 or 1/1000. Thus, methods and techniques which achieve these small errors use high permeability material together with "tuning" or adjusting the assemblies to the required degree of precision.
In order to create a uniform magnetic field, a permanent magnet, steel (or other materials) surrounding the permanent magnet, a shim and a support structure must be effectively designed. A patient must also be able easily to access the magnetic field generating device, and therefore the size of a structure which surrounds the body of the patient must be reduced. A method must also be provided for raising the uniformity of the magnetic field and for matching the magnetic field strength from each magnetic pole with a smaller permanent magnet.
A need exists for a magnetic field generating apparatus for a magnetic resonance imaging (MRI) system with an improved structure in which a permanent magnet shim and a booster shim are included.